This invention relates to a solder reflow oven that uses heated air to heat a workpiece to a temperature effective to reflow solder. More particularly, this invention relates to such solder reflow oven wherein the heated air is directed through a divergent nozzle to extend the distance or time that the workpiece is heated to solder reflow temperatures.
A typical microelectronic assembly comprises electronic components attached to a printed circuit board by solder bonds. Solder bonds are commonly formed using a solder paste comprising solder particles dispersed in a vaporizable vehicle. The solder paste is applied to bond pads on the printed circuit board, and the electronic component arranged in contact with the solder paste. The arrangement is then heated to vaporize the vehicle and to melt and coalesce the solder particles, which is referred to as reflow. Upon cooling, the solder solidifies to bond the electronic component to the printed circuit board.
Solder reflow is carried out by conveying the workpiece, which comprises the arrangement of the electronic component and the printed circuit board with the solder paste, through an oven. An example of an oven is described in U.S. patent application Ser. No. 10/007,485. filed Dec. 3, 2001, and assigned to the assignee of the present invention. Within the oven, the workpiece is initially preheated to a temperature just below the solder melting temperature. The workpiece is then heated using hot air to a temperature effective to reflow the solder. Reheated air is distributed through the reflow zone by an air distribution system that includes an elongated nozzle for directing the air into the workpiece. In accordance with the aforementioned patent application, a nozzle includes vanes for directing the heated air laterally relative to the direction of travel of the workpiece to provide more uniform heating.
In a conventional nozzle, the air outlet is defined by plates that are perpendicular to the workpiece. Referring to FIG. 1, an example is shown of a conventional nozzle 20 for directing heated air into a workpiece 10 comprising an electronic component 12 and a printed circuit board 14. Workpiece 10 is transported through the reflow zone on a conveyer 16 in the direction of arrow 18. The opening from the nozzle is defined by vanes 21 that are perpendicular to direction 18. Air emerging from the nozzle adjacent vanes 21 forms shear layers 22 that are characterized by turbulence that cause mixing of the heated air with surrounding, relatively cooler air. This mixing reduces the temperature within the shear layers below the effective reflow temperature. As a result, the workpiece is heated above the solder reflow temperature only over a relatively short distance between the shear layers. In order to heat workpiece 10 for time sufficient to accomplish the desired reflow, it is necessary to reduce the speed of the conveyor. While it is possible to increase the temperature of the heated air to increase the temperature within the shear zones, this may result in overheating of the workpiece within the region between the shear layers and is not desired. Moreover, the air delivery system is confined by spacial constraints within the oven so that the width of opening cannot be readily increased to lengthen the reflow zone.
Therefore, a need exists for a solder reflow oven having a reflow zone that uses heated air to heat a workpiece to a temperature effective to reflow solder, which is capable of increasing the distance over which the workpiece is heated to solder reflow temperature without interfering with upstream and downstream regions of the oven.
In accordance with this invention, a solder reflow oven comprises a heating zone for heating a workpiece that includes a solder to a temperature effective to reflow the solder. The oven includes a conveyor for transporting the workpiece in a direction sequentially through a pre-reflow zone, the heating zone and a post-reflow zone. A plenum supplies heated air to the heating zone and includes sides that define a plenum opening having a first dimension in the direction of travel. A nozzle is interposed between the plenum opening and the conveyor and receives heated air from the plenum and directs said heated air toward the conveyor. The nozzle includes a front wall and a rear wall in divergent relationship and defining a nozzle opening adjacent the conveyor that has a dimension less than or equal to the dimension of the plenum opening. In this manner, the nozzle is confined within the heating zone and directs shear layers associated with heated air exiting the nozzle opening toward the pre-flow and post-reflow zones, respectively, thereby increasing the distance over which the workpiece is heated to solder reflow temperatures. This is accomplished without the necessity for increasing the temperature of the heated air and while confining the plenum and nozzle to the reflow zone so as not to interfere with equipment or operation in neighboring zones.